At first glance, computerized timekeeping might seem like a cinch. A computer sees time as one thing: A uniform accumulation of discrete moments. The more accurately such moments are captured, the more precise the resulting time.
Humans, on the other hand, have all sorts of quirky notions about how to measure time. We chop it up into weeks, months, days. We stagger it across different time zones. We box it into calendars that correspond only roughly with the orbit of the Earth around the sun. We inject quasi-arbitrary shifts in the form of daylight saving time. And every few millennia, we do away with our timekeeping systems entirely.
For Posix-based computers such as Unix or Linux, time began Jan, 1, 1970, at midnight. The time according to those machines is the number of seconds that have accumulated since. For instance, the Greenwich Mean Time 8:22 p.m., Tuesday, June 10, 2008, translates to 1213129345 in Posix-speak. (NOTE: This number is disputed on the Perl Datetime List
At the SIGAda 2007 conference in Fairfax, Va., one developer bemoaned Linux’s measurement of time only in microseconds, or millionths of a second. Microseconds are fine for most purposes, but the developer had written a program to applications for bugs and wanted to divide time into everfiner slices. He wanted Linux to cycle in nanoseconds, or billionths of a second.
Using algorithms to estimate the offset caused by transmission times, the current version of NTP can synchronize local time with a reference clock to within a few hundred milliseconds, an accuracy that can be maintained by checking the time server every 1,024 seconds. The NTP update would bring the accuracy to within tens of milliseconds and allow as much as 36 hours between checks with the time server.
Despite the improved accuracy, many technologists still say too much ambiguity remains in NTP.
“There is a lot of ambiguity about how you time stamp [an Internet] packet,” said Symmetricom technologist Greg Dowd, speaking at a March IETF meeting in Philadelphia. “I have a tremendously difficult time trying to do high-accuracy, high-stability time transfer” using the NTP protocol, he said.
NIST physicist Till Rosenband is working on an atomic clock based on a pair of ions, one aluminum and the other beryllium. This clock is at least 10 times more accurate than the cesium-based clocks, NIST concluded after a year of measurements. The aluminum ion emits a steady vibration, which is amplified by the beryllium. A femtosecond oscillation of light emitted by a laser records the vibration. A femtosecond, if you’re keeping track, is a quadrillionth of a second.
“The aluminum clock is very accurate because it is insensitive to background magnetic and electric fields, and also to temperature,” Rosenband said. “Accuracy is measured by how well you reproduce the unperturbed frequency of this atom without any background magnetic or electric fields.” Rosenband said standards labs worldwide are in a race to build the next-generation atomic clock.
The new generation of atomic clocks would neither gain nor lose a second in more than 1 billion years — if they could run that long. Such clocks change no more than 1.6 quadrillionth of 1 percent per year. By comparison, the cesium clock can run without gaining or losing a second for only about 80 million years.